1. Field of the Invention
The present invention relates to explosive compositions comprising a sensitized blend of a fuel- and oxidizer-containing aqueous slurry and solid particulate inorganic nitrate, preferably ammonium nitrate (AN), in the form of prills or granules which may be coated with fuel oil (e.g., ANFO).
2. Description of the Prior Art
Ammonium nitrate (AN), an essential ingredient in nearly all commercial explosives, is used predominantly in the form of prills which are small porous pellets mixed with fuel oil and are commonly referred to as ANFO. ANFO products have found extensive use in a wide variety of blasting applications because of economy and convenience. However, because of its low water-resistance, the use of ANFO in bulk form in water-filled boreholes necessitates the de-watering of the holes and lining them with plastic. Moreover, because of its low bulk density, the energy produced from ANFO per unit of volume is low.
In recent years, explosives which comprise a blend of AN prills and a water-in-oil emulsion have captured the interest of blasters owing to the fact that they are able to offer the advantages of high bulk density, blasting energy, and water resistance characteristic of emulsion explosives, while at the same time resulting in cost reductions owing to the lower cost of the AN. Stable explosives of this type are described in U.S. Pat. No. 4,555,278 granted to L. A. Cescon and N. J. Mellet, Jr.
Another type of product which has been recognized as being capable of increasing the bulk density and blasting energy of AN prills when blended therewith is the well-known type known as "slurries" or water gels. These products comprise an inorganic oxidizing salt, usually AN, dissolved, and generally also suspended, in a thickened continuous aqueous phase which also contains one or more fuels and sensitizers. However desirable the possibilities that slurry/prill blends might appear to offer, however, they heretofore have not been well-accepted because of the tendency of the prills to soften and deteriorate into fines, a condition which not only in effect eliminates the presence of the prills as a discrete phase in the blend, but ultimately may destroy the integrity of the slurry structure itself so that the blend is no longer an effective explosive. In prill blends, a discrete prill phase is important on several counts, including better handling characteristics in certain slurry/prill ratios, improved sensitivity, etc.
The above-described prill deterioration problem heretofore has been attacked in various ways. One way has been to package the slurry and ANFO, with a water-proof barrier between the two. For example, U.S. Pat. No. 3,342,132, issued Sept. 19, 1967, to D. S. Partridge, describes a packaged product wherein ANFO, sealed in a polyethylene bag, is placed inside a larger film package containing an aqueous slurry explosive, which surrounds the ANFO package. This patent also describes making waterproof prills according to U.S. Pat. No. 3,148,095, distributing the waterproof prills in an aqueous slurry mixture, and packaging. In this case, the prills are first impregnated with paraffin and then coated with gilsonite and wax. This technique of producing a stable slurry/prill product is disadvantageous because of the added expense of the multiple prill coatings required, and also because it seals in the prill's pores with paraffin, thereby destroying the well-known sensitizing capability of the voids in the prills and yielding a low-density product.
The solution to the problem of prill attack in slurry/prill blends adopted by R. B. Clay in U.S. Pat. No. 4,294,633, issued Oct. 13, 1981, is to avoid aqueous slurries entirely, and to use, instead, slurries based on polar organic liquids, i.e., methanol and ethylene glycol.
In the foregoing patents, slurry/prill blends are made by mixing the prills with with a pre-formed thickened slurry, i.e., a thickened slurry which already contains the AN that is to be present in dissolved form in the product. Such products are true "blends" in the strict sense of the word, inasmuch as they are formed by mixing two components which undergo essentially no compositional change during the mixing. As made, the products contain discrete, well-defined prills in an amount essentially equal to the amount used to form the blend. These products differ from the prill-containing slurries which result from slurry/preparation processes wherein AN, in the form of prills, is added to an aqueous medium for the purpose of producing a saturated solution or liquor. In these products the content, if any, in intact, discrete prills is unpredictable, and hence difficult to control, because of solution-crystallization processes that occur. According to U.S. Pat. No. 3,630,250, issued Dec. 28, 1971, to J. R. Hradel, particulate ammonium nitrate is admixed with water to produce a paste-like (compositions containing 3-12% water) to pumpable (compositions containing 12-25% water) composition containing both dissolved and solid-phase AN. The AN, water, and a particulate metal sensitizer are specifically described as being mixed together by kneading in a plastic bag.
U.S. Pat. No. 4,213,809, issued July 22, 1980, to N. E. Gehrig, describes making an extrudable water gel blasting agent by adding solid AN to an aqueous medium in two steps, both before the hydration of a thickening agent therein occurs. After thickening, a crosslinking agent is added to produce the extrudable gel having the consistency of a thick grout or mortar. The finished product contains a total of 75-85% AN and 10-13% water.
A need exists for products which are sensitized blends formed by combining AN prills, or other inorganic nitrate particulate materials, and fuel- and oxidizer-containing aqueous slurries. More particularly, a need exists for aqueous slurry/prill blends which are stable with respect to prill deterioration when the prills have no waterproof coating.